Silicon Nanostructure Based Heterojunction Photovoltaics For Solar Energy Conversion And Storage

With the rapid change of the global energy strategies,environmental friendly renewable energy attracts wide interests.Solar cells represent an efficient,stable,clean and safe energy source during the past years.Tremendous efforts have been devoted into this research area and various kinds of new type photovoltaic devices were developed besides the commercialized crystalline silicon solar cells,which dominates the market for decades.Smart electronic devices like mobile phones and hand rings are designed lighter and smaller,which puts the power source in a new challenging position.Conventional silicon solar cells have the advantages in large demands of energy,but the rigid configuration of a large and hard panel restrict it in the case of portable or wearable situation.The newly developed devices favor low temperature,non-toxic and low cost processes;and as for the performance,long-term stability,mutli-functions in applications such as being flexible,humidity resistant,transparent and even artistic are needed in addition to the conversion efficiency.In this situation,hybrid solar cells based on the well-developed silicon manufacturing techniques and semiconducting polymers possess unique research interests.Silicon-polymer hybrid solar cells combine the properties of these two different materials,taking advantages of the high stability,carrier mobility,and facilities of crystalline silicon,as well as the modification,band gap adjustable ability and simple solution process of the polymer.The organic-inorganic heterojunction formed through room temperature or low temperature simplifies the fabrication process,reduces the cost,and avoids the negative effects of high temperature induced problem.Silicon nano/micro structures like wires,holes,cones,pyramids can not only reduce the reflection of the surface,but also provide a larger junction area which benefits the separation of carrier at the interface,and at the meantime the reduced reflection allows less materials volume used for light absorption and lower quality of silicon for carrier transport.It is important to investigate this kind of silicon nanostructure-based hybrid solar cells due to their simple fabrication process and high efficiency.But further improvement of the performance is limited by the serious surface recombination rate on the nano-surface.In this study,silicon nano-honeycomb structures and nanowire array structures were fabricated on the surface,and then optimized the device structure.Finally an integrated solar cell-supercapacitor self-powered system was demonstrated.The influences of surface engineering,interface passivation and structures on the device were systematically investigated.Followings are the main content of this work:1.A novel nano-honeycomb array structure was fabricated on silicon by combing nanosphere lithography with plasma etching,which reduced the light reflectance to as low as 5%.Trimethylanilinium hydroxide（TMAH）solution was applied to polish the surface roughness of the nanostructure as well as lower the surface-to-volume ratio,resulting in a reduced carrier recombination rate at the interface.The device processed in TMAH for 60s exhibited a power conversion efficiency（PCE）of 12.79%,showing 59%improvement compared with the controlled sample.2.A layer of molybdenum trioxide（MoO₃）film with high reflective index on poly（3,4-ethylenedioxythiophene）:poly（stylenesulfonate）（PEDOT:PSS）film formed a double-layer antireflection coating on Si,and the reflection was moderated through varying the thickness of the MoO₃ film while the thickness of polymer was fixed.The high work function transition oxide layer could also induce an inversion layer on the silicon beneath it.Surface potential scanning showed a lower surface potential with MoO₃ layer on PEDOT:PSS,and capacitance-voltage measurements indicated both the built-in potential and the width of the depletion layer obviously increased.The inversion layer turned the minority carrier into majority carrier in the depletion layer,reducing the carrier transport resistance.The surface states of silicon were occupied by these excess minority carrier,which eliminated their chance to be recombination center,thus resulting in less carrier lost and higher carrier collection efficiency.With 15 nm-thick MoO₃ film deposited on the optimized device,a highest PCE of 13.8%was achieved with an open-circuit voltage of630 mV,a short-circuit current density of 29.2 mA/cm² and a fill factor of 74.9%.Compared with the best device without MoO₃ layer,the as-process device showed 11%enhancement in the current,21%enhancement in PCE,respectively,with 30 mV improvement in voltage,which demonstrated the superior effect of this facile process.3.Large area,highly ordered vertical silicon nanowire array were fabricated through nanosphere lithography and metal assisted chemical etching process.The period,diameter and length of the nanowire could be controlled by varying the etching condition,thus tuning the light properties.A methylation process was carried out to reduce the surface roughness of the nanowire as well as grew a layer of silicon-methyl group instead of the silicon-hydrogen bond,which was unstable in the ambient atmosphere,thus improving the minority carrier lifetime by passivation effect.A PCE of 13.39%was achieved in the silicon nanowire array-PEDOT:PSS hybrid solar cells.4.By using a thin layer of thermal evaporated Ti film and electro-chemically synthesized polypyrrole as the connection layer,we developed an integrated self-power system with a highest reported total conversion and storage efficiency of 10.5%that could simultaneously convert the solar energy by the hybrid solar cell and store the electricity in the supercapacitor.A flexible self-power system was also demonstrated to show its potential application in portable or wearable electronics.